Drug Toxicity and Overdose Management

• Dose-dependent toxicity and receptor saturation: Most drugs exhibit dose-response relationships governed by the law of mass action, where clinical effects escalate predictably with serum concentration. Toxicity emerges when drug concentration exceeds the therapeutic window, overwhelming normal buffering mechanisms and saturating target receptors. For example, acetaminophen toxicity occurs when hepatic glutathione (GSH) becomes depleted by N-acetyl-p-benzoquinoneimine (NAPQI), the toxic metabolite; once GSH falls below ~30% of baseline, NAPQI accumulates and covalently binds hepatocellular proteins, causing hepatonecrosis. Similarly, opioid overdose produces profound respiratory depression through μ-receptor agonism in the respiratory centers; excessive activation suppresses central chemoreceptor response to hypercapnia and hypoxia, leading to apnea and hypoxemic cardiopulmonary arrest. The molecular basis involves G-protein-coupled receptor activation with downstream inhibition of adenylyl cyclase and reduced cAMP, diminishing neuronal excitability in critical brainstem nuclei.

• Metabolic pathway saturation and toxic metabolite accumulation: Phase I (oxidative), Phase II (conjugative), and Phase III (transport) metabolism follow Michaelis-Menten kinetics; at high substrate concentrations exceeding enzyme Km values, metabolism shifts from first-order to zero-order kinetics, and clearance becomes rate-limited. Salicylate overdose exemplifies this principle: at therapeutic doses (<100 mg/kg), salicylate is rapidly conjugated; at toxic doses (>300 mg/kg), conjugation becomes saturated, increasing unmetabolized salicylate half-life from 2-3 hours to 15-30 hours. Accumulated salicylate uncouples oxidative phosphorylation in mitochondria, directly stimulates the respiratory center (producing respiratory alkalosis), and generates metabolic acidosis through impaired cellular respiration—creating a mixed acid-base derangement. Methanol and ethylene glycol undergo hepatic conversion by alcohol dehydrogenase to toxic aldehydes and organic acids (formaldehyde → formic acid; glycolaldehyde → glycolic acid) that produce profound metabolic acidosis with elevated anion gap, ocular toxicity (methanol), and acute kidney injury from crystalline nephropathy (ethylene glycol).

• Organ-specific tissue accumulation and cellular injury: Certain drugs accumulate preferentially in specific tissues due to lipophilicity, protein binding, or active transport, creating pharmacologically active concentrations far exceeding serum levels. Tricyclic antidepressants (TCAs) are highly lipophilic with large volumes of distribution (5-20 L/kg); they block cardiac sodium channels (Class IA antiarrhythmic effect) causing QRS prolongation, atrioventricular conduction delay, and polymorphic ventricular tachycardia, particularly in the setting of alkalemia (which enhances sodium channel blockade). Digoxin accumulates in skeletal muscle and becomes toxic when serum levels exceed 2-3 ng/mL in chronic toxicity or >4-6 ng/mL acutely; toxicity stems from inhibition of the Na+/K+-ATPase pump, which increases intracellular calcium, promoting automaticity and triggered activity that produce atrial fibrillation with rapid ventricular response and diverse arrhythmias. Lithium accumulates in the central nervous system, thyroid, and kidney; chronic toxicity causes nephrogenic diabetes insipidus through damage to the collecting duct, producing nephrogenic diabetes insipidus, tremor, confusion, and irreversible cerebellar atrophy.

• Oxidative stress and reactive oxygen species generation: Many drugs undergo metabolic transformation to reactive intermediates that generate free radicals and deplete cellular antioxidant defenses. Acetaminophen's NAPQI binds glutathione and depletes GSH; once GSH falls critically low, NAPQI-protein adducts accumulate and trigger oxidative stress, mitochondrial dysfunction, and hepatocyte necrosis through mechanisms including JNK phosphorylation and opening of the mitochondrial permeability transition pore. Stimulant drugs (cocaine, amphetamines) increase catecholamine production, promoting autooxidation of dopamine and norepinephrine to quinone species that generate reactive oxygen species (ROS) and overwhelm antioxidant capacity in the nucleus accumbens and substantia nigra, contributing to acute cardiovascular toxicity (myocarditis, coronary vasospasm) and chronic neurotoxicity. Iron overdose produces ROS through Fenton chemistry (Fe²⁺ + H₂O₂ → Fe³⁺ + OH⁻ + OH•), causing corrosive injury to the gastrointestinal mucosa, systemic iron-catalyzed lipid peroxidation, hepatotoxicity, and myocardial depression.

• Cardiovascular electrophysiologic abnormalities and arrhythmogenic mechanisms: Multiple drug classes directly affect cardiac action potential generation and propagation. Antiarrhythmic drugs (Classes I-IV), tricyclic antidepressants, and antipsychotics in overdose block sodium channels, prolonging depolarization and slowing conduction velocity (manifested as QRS widening on ECG). Calcium channel blockers suppress atrioventricular nodal conduction and myocardial contractility, producing bradycardia, heart block, and cardiogenic shock. Beta-blockers similarly impair contractility and heart rate, though without QRS widening. Long-actingQT-prolonging drugs (macrolide antibiotics, antipsychotics, methadone) block cardiac potassium channels, delaying repolarization and creating the electrophysiologic substrate for torsades de pointes (a polymorphic ventricular tachycardia associated with QT prolongation), particularly in the setting of hypokalemia or hypomagnesemia.

• Central nervous system depression, seizure threshold, and encephalopathy: Sedative-hypnotics (benzodiazepines, barbiturates), opioids, ethanol, and antiepileptic drugs (at toxic levels) enhance GABAergic inhibition or suppress glutamatergic excitation, producing dose-dependent depression of consciousness progressing from sedation to unresponsiveness, loss of protective airway reflexes, and respiratory depression. Conversely, abrupt withdrawal from CNS depressants or overdose with stimulants (cocaine, amphetamines, MDMA) lowers seizure threshold by increasing neuronal excitability or causing hyperthermia, electrolyte derangements, or cerebral edema. Isoniazid overdose directly inhibits pyridoxal phosphate (vitamin B6) and depletes GABA synthesis, producing refractory seizures unresponsive to standard anticonvulsants. Theophylline overdose blocks adenosine receptors and inhibits phosphodiesterase, increasing cAMP and causing tachyarrhythmias, seizures, and metabolic derangements.

• Metabolic derangements: acidosis, alkalosis, and electrolyte abnormalities: Different toxins produce characteristic acid-base patterns. Salicylates, methanol, ethylene glycol, metformin (lactic acidosis), cyanide, and isoniazid produce metabolic acidosis by generating organic acids or impairing aerobic metabolism. Theophylline and stimulants produce metabolic acidosis via increased cellular metabolism and lactate production. Acetazolamide and topiramate cause metabolic acidosis through urinary bicarbonate wasting. Tricyclic antidepressants can produce acidosis through tissue hypoxia and shock. Electrolyte abnormalities include hyponatremia from SIADH (carbamazepine, oxcarbazepine, SSRIs), hypokalemia from beta-agonist and theophylline toxicity (via β2-mediated K+ shift into cells), hyperkalemia from ACE inhibitors, potassium-sparing diuretics, and renal failure, and hypophosphatemia from phosphate-binding antacids or alcoholic ketoacidosis.

• Unintentional overdose—pharmaceutical medication errors: Accidental ingestion of supratherapeutic doses represents the most common scenario in pediatric patients and results from dosing miscalculation, confusion between different formulations (immediate-release vs. extended-release), or parental administration errors. Acetaminophen toxicity frequently results from inadvertent overdose when patients consume multiple combination products containing acetaminophen without recognizing cumulative dosing (e.g., acetaminophen + oxycodone, acetaminophen + hydrocodone). Similarly, iron overdose in young children occurs when pediatric-strength vitamins containing iron are accessed. The elderly are at heightened risk for unintentional overdose due to polypharmacy, renal/hepatic impairment reducing drug clearance, cognitive decline, and reduced medication adherence awareness.

• Intentional self-poisoning and suicidal overdose: Deliberate ingestion of drugs in supratherapeutic amounts to cause harm or death represents the second most common scenario. Sedative-hypnotics (benzodiazepines, barbiturates), opioids, tricyclic antidepressants, and salicylates are among the most frequently used agents for self-harm. Psychiatric history, recent psychosocial stressors, access to medications, and social isolation are major risk factors. Self-poisoning often involves ingestion of multiple agents (polysubstance overdose), which complicates toxidrome recognition and management.

• Substance abuse and illicit drug use: The ongoing opioid epidemic has made heroin and illicitly-manufactured fentanyl (and fentanyl analogs like carfentanil) leading causes of overdose death, particularly in younger adults. Contamination of street drugs with high-potency fentanyl substantially increases overdose risk. Cocaine and methamphetamine use, particularly in binges, produce acute toxicity with cardiac (myocarditis, arrhythmias, coronary vasospasm) and neurologic (seizure, intracranial hemorrhage) complications. Synthetic cathinones ("bath salts"), synthetic cannabinoids, and other novel psychoactive substances carry unpredictable potency and toxicity profiles.

• Dosing errors and medication administration mistakes: Healthcare-provider errors in medication calculation, route of administration, or infusion rate cause significant morbidity. Insulin overdose from miscalculation produces profound hypoglycemia and neurologic injury. Digoxin toxicity arises from therapeutic failure to account for renal impairment or drug-drug interactions (e.g., concurrent verapamil, amiodarone increasing digoxin levels). Chemotherapy agents possess narrow therapeutic windows; errors in dosing (especially when body surface area is miscalculated) lead to acute toxicity and organ failure. Heparin overdose from unintentional bolus administration causes life-threatening hemorrhage.

• Drug-drug interactions impairing elimination: Inhibition of hepatic cytochrome P450 enzymes or renal clearance pathways by concurrent medications elevates drug levels of co-administered agents. Macrolide antibiotics, azole antifungals, protease inhibitors, and SSRIs inhibit CYP3A4, elevating levels of drugs metabolized by this pathway (e.g., terfenadine, astemizole, pimozide—all now removed from market due to QT prolongation). Ketoconazole and erythromycin elevate digoxin levels through CYP3A4 inhibition and P-glycoprotein inhibition. NSAIDs, ACE inhibitors, and diuretics impair renal function, reducing clearance of drugs like lithium, metformin, and aminoglycosides, precipitating toxicity.

• Pharmacogenomic variation in drug metabolism: Genetic polymorphisms in metabolizing enzymes create substantial inter-individual variation in drug clearance. CYP2D6 poor metabolizers accumulate tricyclic antidepressants and codeine to toxic levels at standard doses, increasing risk of cardiotoxicity and respiratory depression. N-acetyl transferase 2 (NAT2) slow acetylators accumulate isoniazid, increasing seizure risk. Thiopurine methyltransferase (TPMT) deficiency impairs metabolism of azathioprine and 6-mercaptopurine, causing severe myelosuppression and infection.

• Renal and hepatic impairment: Chronic kidney disease reduces clearance of renally-excreted drugs (digoxin, lithium, aminoglycosides, NSAIDs), while cirrhosis, acute liver failure, and hepatitis reduce metabolism of hepatically-cleared drugs and worsen encephalopathy from ammonia accumulation. Acetaminophen toxicity is accelerated in patients with baseline liver disease or heavy alcohol use due to hepatic GSH depletion and CYP2E1 induction.

• Alcohol and substance use history: Chronic alcohol use induces CYP450 enzymes (particularly CYP2E1), accelerating metabolism of certain drugs (e.g., acetaminophen) while simultaneously depleting hepatic GSH, rendering patients at heightened risk for hepatotoxicity from acetaminophen at doses that would be safe in non-alcoholics. Alcohol also lowers seizure threshold and impairs judgment, increasing risk of polysubstance overdose.

• Opioid toxicity (CNS depression, respiratory depression, miosis): The classic opioid toxidrome consists of decreased consciousness ranging from drowsiness to unresponsiveness, markedly depressed respiratory rate (bradypnea), and pinpoint pupils (miosis) resulting from μ-receptor agonism in the locus coeruleus and Edinger-Westphal nucleus. Respiratory depression represents the life-threatening component, as hypoventilation produces hypercapnia and hypoxemia, progressing to apneic arrest requiring mechanical ventilation. Pulmonary edema ("narcotic pulmonary edema") may develop acutely, mediated by increased pulmonary vascular permeability and altered hemodynamics. Miosis is not universally present if the opioid is combined with anticholinergic agents (e.g., diphenoxylate) or in anoxic brain injury.

• Sedative-hypnotic toxicity (progressive CNS depression, ataxia, respiratory depression): Benzodiazepines, barbiturates, zolpidem, and other sedatives produce dose-dependent depression of consciousness, beginning with mild sedation, progressing through ataxia and dysarthria, to loss of protective airway reflexes (inability to maintain oxygenation/ventilation). Respiratory depression occurs but is typically less severe than with opioids alone; however, combination with opioids produces profound respiratory depression. Hyporeflexia and hypotonia develop with higher doses. Pupil size is typically normal or midposition. Chronic benzodiazepine use followed by abrupt cessation produces a withdrawal syndrome (see Complications section).

• Anticholinergic toxidrome (mydriasis, tachycardia, hyperthermia, delirium, dry mucous membranes, urinary retention): Tricyclic antidepressants, atropine, antihistamines, antipsychotics, and antiparkinsonian drugs block musc

Immediate bedside tests

• Point-of-care glucose: hypoglycemia (insulin, sulfonylurea, ethanol, salicylate) mimics any toxidrome and must be excluded before anything else.
• 12-lead ECG: the single highest-yield toxicology test. QRS >100 ms after sodium-channel blocker (TCA, diphenhydramine, cocaine, flecainide) ingestion predicts seizure; QRS >160 ms predicts ventricular dysrhythmia. A terminal R wave in aVR ≥3 mm is the classic TCA finding. Measure QTc for torsades risk.
• Venous or arterial blood gas with electrolytes: defines the acid–base pattern. Anion-gap metabolic acidosis suggests methanol, salicylate, iron, isoniazid, metformin/lactate, or ethylene glycol; mixed respiratory alkalosis plus anion-gap acidosis in an adult is salicylate until disproven.

Confirmatory quantitative levels

• Serum acetaminophen and salicylate on every intentional overdose: both are common, initially silent, and have antidotes.
• Rumack–Matthew nomogram: plotted at ≥4 hours post-ingestion for a single acute acetaminophen ingestion; a level on or above the 150 mcg/mL at 4 hours treatment line mandates acetylcysteine. The nomogram is invalid for staggered, chronic, or unknown-time ingestions — treat on clinical grounds.
• Osmolal gap: a gap above roughly 10 mOsm/kg supports toxic alcohol before the parent alcohol is metabolized; a normal gap late does not exclude it, since the gap converts to an anion gap as acids form. Ethylene glycol may show calcium oxalate crystalluria and urine fluorescence.
• Drug-specific levels: digoxin, lithium, iron, theophylline, valproate, carboxyhemoglobin, methemoglobin (chocolate-brown blood, saturation gap between pulse oximetry and measured PaO₂).
• Urine immunoassay drug screens rarely change management — false negatives for fentanyl and synthetic opioids are routine.

Stabilisation first (ABCs before antidotes)

• Airway and ventilation: intubate for lost airway reflexes or apnea; hypoxic arrest, not the drug itself, kills most opioid patients.
• Empiric reversal agents: opioid antagonist — naloxone 0.4 mg IV, titrated upward (or 4 mg intranasal in the field), aiming for adequate respiration, not full arousal. Give dextrose for documented hypoglycemia and thiamine in malnourished patients.
• Seizures: benzodiazepines (lorazepam) are first-line for nearly all toxic seizures; phenytoin is ineffective and potentially harmful in sodium-channel blockade.

Decontamination and enhanced elimination

• Activated charcoal: per the AACT/EAPCCT position statement, consider within about 1 hour of a potentially toxic ingestion with a protected airway. Contraindicated with depressed consciousness and an unprotected airway, caustics, and hydrocarbons; ineffective for iron, lithium, and alcohols. Syrup of ipecac and routine gastric lavage are no longer recommended.
• Whole-bowel irrigation with polyethylene glycol for iron, sustained-release verapamil or lithium, and body packers.
• Urinary alkalinization with sodium bicarbonate for salicylate; hemodialysis per EXTRIP workgroup recommendations for severe salicylate, lithium, methanol, ethylene glycol, metformin-associated lactic acidosis, and valproate.

Specific antidotes

• Acetylcysteine (IV) for acetaminophen — glutathione precursor; near-complete hepatoprotection if started within 8 hours.
• Fomepizole, an alcohol dehydrogenase inhibitor, for methanol/ethylene glycol, with dialysis for acidosis or end-organ injury.
• Sodium bicarbonate boluses for TCA-induced QRS widening (sodium load plus alkalemia relieves channel blockade).
• Digoxin-specific Fab for hyperkalemia or dysrhythmia from digoxin.
• Glucagon and high-dose insulin euglycemic therapy for beta-blocker and calcium channel blocker shock, with IV calcium; the 2023 AHA Focused Update on poisoning endorses high-dose insulin, and VA-ECMO for refractory shock.
• Atropine plus pralidoxime (organophosphates), pyridoxine (isoniazid), hydroxocobalamin (cyanide), deferoxamine (iron), IV lipid emulsion (local anesthetic systemic toxicity, per ASRA).

Airway and pulmonary (emergencies)

• Aspiration pneumonitis/ARDS: obtunded patients lose gag reflex; signalled by new hypoxemia and bilateral infiltrates after an unwitnessed down-time. Charcoal aspiration causes a particularly severe chemical pneumonitis.
• Noncardiogenic pulmonary edema: seen with opioids and salicylates from increased pulmonary capillary permeability; frothy sputum with hypoxemia despite normal cardiac filling pressures.

Cardiovascular (emergencies)

• Ventricular dysrhythmia: sodium-channel blockade widens QRS and produces monomorphic VT; potassium-channel blockade prolongs QT and produces torsades de pointes, treated with IV magnesium. Ventricular fibrillation/pulseless VT are the shockable rhythms in toxin-induced arrest.
• Cardiogenic shock and bradyasystole: calcium channel blocker, beta-blocker, and digoxin toxicity; hallmark is hypotension with an inappropriately slow rate unresponsive to fluids.

Metabolic and renal

• Rhabdomyolysis → acute kidney injury: from prolonged immobility, agitation, seizures, or hyperthermia; markedly elevated creatine kinase with pigmented granular casts and a urine dipstick positive for blood without red cells. Compartment syndrome may coexist.
• Hyperthermia: sympathomimetic, anticholinergic, serotonin, and neuroleptic malignant syndromes; core temperature above roughly 40°C is an emergency requiring aggressive external cooling and sedation, not antipyretics.
• Hepatic failure: acetaminophen — transaminases in the thousands, then rising INR and encephalopathy; refer to a transplant center per AASLD guidance.

Neurologic

• Anoxic brain injury after opioid or sedative apnea, and status epilepticus from isoniazid, bupropion, or theophylline.

Treatment-related

• Anaphylactoid reaction to IV acetylcysteine: histamine-mediated flushing, urticaria, bronchospasm during the loading infusion; slow or pause the infusion and treat with antihistamines rather than abandoning therapy.
• Flumazenil-precipitated seizures and naloxone-precipitated withdrawal (agitation, vomiting with aspiration risk).
• High-dose insulin therapy: hypoglycemia and hypokalemia requiring intensive monitoring.
• Sodium bicarbonate: hypernatremia, hypokalemia, and overshoot alkalemia.
• Deferoxamine: hypotension with rapid infusion, ARDS with prolonged infusion, and Yersinia sepsis.
• Withdrawal syndromes from abrupt cessation of alcohol, benzodiazepines, or baclofen — autonomic instability and seizures are life-threatening.

• Check a glucose and an ECG before anything else. In an undifferentiated altered patient, the single best next step is fingerstick glucose; in a suspected pill ingestion it is a 12-lead ECG. Both are cheap, immediate, and change management.
• QRS >100 ms after an antidepressant overdose = sodium-channel blockade = give sodium bicarbonate. The terminal R wave in aVR is the classic ECG buzzword. Do not reach for physostigmine because the patient looks anticholinergic — in TCA toxicity it can cause asystole.
• Mixed respiratory alkalosis with anion-gap metabolic acidosis is salicylate. Tinnitus, hyperpnea, fever, and diaphoresis complete the picture. Alkalinize the urine; dialyze for severe toxicity, altered mental status, or pulmonary edema.
• The Rumack–Matthew nomogram only works for a single acute acetaminophen ingestion with a known time, plotted at 4 hours or later. If the timing is unknown, staggered, or chronic, start acetylcysteine empirically rather than waiting for a level. Full protection requires initiation within about 8 hours.
• Osmolal gap early, anion gap late in toxic alcohols: the gap converts as alcohol dehydrogenase generates acids. Calcium oxalate crystals and renal failure = ethylene glycol; visual loss and a "snowstorm" = methanol. Fomepizole blocks the enzyme; dialysis removes the toxin.
• Avoid flumazenil in the undifferentiated overdose. Precipitating benzodiazepine withdrawal in a co-ingestion (especially with a TCA) can produce seizures you can no longer treat with benzodiazepines. The common distractor is using it as the "antidote pair" to naloxone.
• Hyperkalemia after digoxin ingestion predicts mortality and mandates digoxin-specific Fab, not standard potassium-lowering measures alone. Chronic toxicity presents with nausea, confusion, and yellow-green visual halos at modestly elevated levels, particularly with renal impairment or diuretic-induced hypokalemia.
• A negative urine drug screen does not exclude opioid overdose — fentanyl and synthetic opioids are not detected by standard immunoassays. Treat the clinical toxidrome (miosis, bradypnea, coma) with naloxone.